This invention relates to a method and plant for separating air.
The most important method commercially for separating air is by rectification. In typical air rectification processes there are performed the steps of compressing a stream of air, purifying the resulting stream of compressed air by removing water vapour and carbon dioxide from it, precooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure column and a lower pressure column, i.e. one of the two columns operates at a higher pressure than the other. Most of the incoming air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and a nitrogen vapour. The nitrogen vapour is condensed. Part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column and is used to form a feed stream to the lower pressure column. Typically, the oxygen-enriched liquid stream is sub-cooled and introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid air is separated into substantially pure oxygen and nitrogen in the lower pressure column. Gaseous oxygen and nitrogen products are taken from the lower pressure column and typically form the returning streams against which the incoming air is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure column through a throttling valve. An upward flow of vapour through the lower pressure column from its bottom is created by reboiling liquid oxygen. The reboiling is carried out by heat exchanging the liquid oxygen at the bottom of the lower pressure column with nitrogen from the higher pressure column. As a result, the condensed nitrogen vapour is formed.
A local maximum concentration of argon is created at an intermediate level of the lower pressure column beneath that at which the oxygen-enriched liquid air is introduced. If it is desired to produce an argon product, a stream of argon-enriched oxygen vapour is taken from a vicinity of the lower pressure column where the argon concentration is typically in the range of 5 to 15% by volume of argon, and is introduced into a bottom region of a side column in which an argon product is separated therefrom. Reflux for the argon column is provided by a condenser at the head of the column. The condenser is cooled by at least part of the oxygen-enriched liquid air upstream of the introduction of such liquid air into the lower pressure column.
In such a process the oxygen and nitrogen products are typically taken in gaseous state. It is however frequently required to take a proportion of the oxygen and nitrogen products in liquid state. One example is the process described in EP-A-0 576 3 14A. The rate of providing refrigeration for the process may readily be set so as to meet any requirements for providing liquid products. However, the mere provision of refrigeration work in order to meet that work required to liquefy the products does not ensure of itself a satisfactory process. In particular, there is a problem that the greater the proportion of oxygen and nitrogen produced in liquid state as products, the more the upper sections of the lower pressure rectification columns tend to be starved of reflux with the result that the concentration of the argon in the argon-enriched oxygen feed to the argon rectification column falls and the yield of argon drops. In practice, these considerations have the consequence that there is a practical ceiling placed on the proportion of the products of the air separation plant that can be taken in liquid state. It is an aim of the present invention to provide a method and plant which enable this limit to be raised.